A common gear motor assembly includes an electric motor and a gearbox. A worm is fixed to or formed on an output shaft of the motor. A worm wheel is mounted in the gearbox on a stationary shaft and arranged to be driven by the worm. The gearbox further includes an output gear. The output gear coaxially connects with the worm wheel and rotates with the worm wheel to thereby drive an external device. A housing of the gearbox includes a chamber for mounting of the worm wheel and an opening in communication with the chamber allowing the output gear to extend there through. A cover is used to seal the opening. A conventional cover includes an outer annular portion made from plastic and an inner annular portion made from rubber. The outer annular portion is mounted to the gearbox housing, and the inner annular portion provides a seal between an outer cylindrical surface of a boss of the worm wheel and an inner bore of the cover. One disadvantage of this conventional cover is that, lateral deviation of the output gear can easily occur under the influence of an external force, especially a radial force, which may lead to flexural deflection of the stationary shaft to which the worm wheel is mounted and hence the generation of vibrations and noises.